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Improving Slow Start

Improving Slow Start. InKwan Yu. Slow Start?. Slow Start Grows the congestion window size exponentially. Terminates when ssthresh (64K) is reached or a packet is lost Three cases of Slow Start Initial : when a TCP connection is established Idle : packets are not sent for a few minutes

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Improving Slow Start

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  1. Improving Slow Start InKwan Yu

  2. Slow Start? • Slow Start • Grows the congestion window size exponentially. • Terminates when ssthresh (64K) is reached or a packet is lost • Three cases of Slow Start • Initial : when a TCP connection is established • Idle : packets are not sent for a few minutes • Timeout : transmission timeout

  3. Related Work • Estimating Bandwidth • Hoe’s Approach • First packet pair to estimate ssthresh • RTT / Inter packet gap = cwd (initial congestion window size or ssthresh) Network Inter Packet Gap s t

  4. Related Work • Caching • Source has cache of ssthresh values of destinations • When a new connection is from a cached destination, use the ssthresh of the cache.

  5. Related Work • Hu’s Method • Congestion : input inter packet gap is smaller than output packet gap • No congestion : input inter packet gap is larger • Binary search using about 3 to 4 rounds

  6. Related Work • Hu’s Method

  7. Goals • Faster Start • In a large bandwidth*delay path, slow start is not fast enough due to TCP’s self clocking • Better ssthresh value • Overestimated ssthresh value will cause packet drops

  8. New Methods • History based • With fair TCP, the equal amount of bandwidth will be shared 1Mb s t 512Kb s t 512Kb

  9. New Methods • History based • When there are already k connections, a new connection will have B*1/(k+1) share from B total available bandwidth 1/k …… …… s t 1/k ?

  10. New Methods • History based • Extension to a subnet, inside of which delay is negligible and there is no congestion • There is a common path from the source to the subnet t Subnet t s Host Host t

  11. New Methods • Estimation based • Weakness of Hu’s method • The binary search method can be misleading with the first few errors • The cwd growth rate is to the 2nd power (slow) • The delayed acknowledgement • Modification • Regression based method • Confidence interval • Sequence numbers of packet pair are reversed

  12. New Methods • Estimation based y=x Arriving Inter Packet Gap (ACK gap) Departing Inter Packet Gap

  13. Reference • Ningning Hu, P. Steenkiste, Improving TCP startup performance using active measurements: algorithm and evaluation, ICNP 2003. • J. Hoe, Improving the start-up behavior of a congestion control scheme for TCP, SIGCOMM 1996.

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